1. Field of the invention
The invention concerns a planar microwave receive and/or transmit array antenna.
It is more particularly concerned with a dual polarization and dual beam antenna.
It also concerns the application of an antenna of this kind to individual reception from two geostationary television satellites, also known as DTH (Direct To Home) satellites, for example in the X band (12.1 GHz).
2. Description of the Prior art
It is clear that dual beam antennas are highly beneficial in many applications such as reception from two satellites in different orbital positions. This applies to pairs of satellites such as ASTRA and TELECOM, ASTRA and EUTELSAT, etc.
The standard technique uses parabolic antennas having two receive heads offset relative to the focusing point, each adapted to receive one of the beams. It is also possible to use motorized parabolic antennas enabling reception from two or more satellites, but these are of higher cost.
This type of antenna is naturally large, even if the high radiated power of recent satellites has made it possible to reduce the overall dimensions significantly. The esthetics of such antennas have also been criticized.
An interesting alternative to this type of antenna could be planar array antennas, essentially based on multilayer printed circuit boards, and more particularly antennas of the slot radiating element type.
However, despite considerable research and development effort, there are as yet no dual beam and dual polarization planar antennas for consumer applications of the above type that are both economical and suitable for mass production.
Furthermore, this type of antenna must have a high efficiency and a wide bandwidth to cover the bandwidth of the satellites to be received (typically 20% of the combined bandwidth).
Many planar antennas have been proposed. However, these are either projects that did not get beyond the laboratory stage (experimental antennas) or antennas for professional use, for example for radar applications.
There follows a non-exhaustive list of such antennas:
An experimental radial line dual beam type planar antenna is proposed in the article by Jun-Ichi Takada et al: "A Dual Beam-Polarized Radial Line Slot Antenna" published in "IEE Antennas and Propagation Society International Symposium", 1993, pages 1624-1627. However, this antenna provides only one polarization per beam. It should also be noted that a radial line antenna provides only a narrow bandwidth (less than 5% of the combined bandwidth). Furthermore, the structure adopted has inherent tight manufacturing constraints, even in a single beam version. The problems are naturally more severe in a dual beam version.
Inclined beams for array type antennas can be generated by feeding the radiating elements of such antennas with signals having a progressive phase-shift to match the phase differences of the inclined wave received by each radiating element.
This phase-shift can be obtained in the circuit feeding the array by many methods, for example using phase-shifters, delay lines, etc. These methods are well known in the context of radar or space transmission applications.
In the case of fixed beam passive arrays, the phase-shift can be obtained by appropriate modification of the length of the feed lines, as described, for example, in "Handbook of Microstrip Antennas", R. P. OWENS, J. R. James Hall, P. S. Hall, IEE, Vol. II, 1989, Peter Peregrinus, London, pages 825-843 and 856-866 (see more particularly (figure 14.9).
For multiple beams, a plurality of radiating element phase excitations are required, using beam forming networks. Blass or Butler matrices may be used for this, for example.
These methods are relatively simple to implement in the case of linear arrays, but not in the case of planar two-dimensional arrays. It becomes very difficult to lay out the required circuits, feed lines, power dividers, hybrid circuits, etc, especially when there are hundreds of radiating elements, as in large size array antennas suitable for receiving from direct broadcast television satellites. These components have to be inserted between the radiating elements.
Moreover, in this type of application the series feeds described in the aforementioned book (figure 14.33) are not suitable anyway since the bandwidth is limited for large size arrays.
Other types of feed have been proposed, for example in European patent application EP-A-0 252 779 (Emmanuel RAMMOS), more particularly with reference to figure 16. The structure described (length of the excitation line and the output connectors) provides a large bandwidth. However, the antenna described provides dual polarization or a dual beam but not both at once.
Finally, the combination of series feeds and matrices or hybrid circuits is also possible. One such combination is disclosed in the aforementioned book, more particularly with reference to figure 14.35, but it does not provide sufficient bandwidth for the preferred application of the invention. What is more, it is in practise restricted to relatively small arrays.
One possible solution, meeting all the requirements of the preferred application of the invention and solving all the problems that have been raised, would be to implement transitions between the radiating elements and multilayer feed arrays. This technique has been used in the case of generation of dual polarization for vertical transition arrays. It is described in the aforementioned book, with reference to figure 14.32.
However, it should be noted that transition radiating elements are in practise ruled out for antennas for receiving from direct broadcast television satellites. They have a narrow bandwidth, require the use of high-performance dielectric materials and imply very tight manufacturing tolerances. Even in the case of dual polarization at only two levels, feed transition arrays are not suitable for an antenna for receiving from satellites. Arrays of this kind have not been marketed. A fortiori, at the manufacturing stage, this type of transition array is not compatible with multilayer feeds, without having recourse to vertical transitions, soldering, etc, which are highly complex and costly.
The teachings that can be drawn from prior art antennas and studies we have carried out show that, realistically, an antenna for "consumer" requirements must be derived in a simple manner from an existing planar antenna design. It must additionally offer a dual beam and dual polarization reception facility for it to be applied to receiving from direct broadcast television satellites. More generally, it must offer a transmit and/or receive capability having this two-fold property for less specific applications.
An object of the invention is therefore to provide an antenna of the aforementioned type, compatible with all the stated requirements, in particular low manufacturing cost, easy manufacture with no need to comply with tight tolerances, high efficiency and large bandwidth. It additionally offers the two-fold property just referred to.
To this end, the antenna of the invention retains most of the features of the structure adopted for prior art planar antennas, advantageously those of the antenna described in the previously mentioned European patent application EP-A-0 252 779.
This latter antenna, in an embodiment with provision for dual polarization, comprises slot radiating elements. To this end, a stack of three metal ground plates is provided, having openings and a pair of suspended printed circuit technology microstrips. The microstrips are disposed between the ground plates, one for the vertical polarization and the other for the horizontal polarization.
As described in detail hereinafter, to achieve the object of the invention it is sufficient to add to this basic structure a pair of feed circuits, one on top of the sandwich formed by the aforementioned three plates and the other under it.
In a preferred embodiment of the invention each pair of microstrips (or more generally transmission lines) has the dual polarization facility.
By virtue of these arrangements, the antenna of the invention has a dual beam and dual polarization receive and/or transmit capability enabling it to receive from and/or transmit to two different directions an electromagnetic signal having two different polarizations.